The present invention relates to developing apparatuses for use in an electrophotographic copying machine or the like, and more specifically, to an improved developing apparatus in which a toner surface is provided spaced opposite an electrostatic latent image surface to define a very small gap therebetween, and an A.C. bias voltage is applied across the gap to let toner fly and attach to the latent image portion.
For an electrophotographic copying apparatus widely applied to electronic copying machines, facsimile equipment, printers and the like, there has been so far employed mainly a developing method such as a cascade method or a magnetic brush method. Recently, there has been an increasing demand for putting color recording to practical use. In order to meet such demand, studies have been made to develop an image on photosensitive material allowing the superimposed development of an unfixed image on a non-contact basis. This developing method is generally called a non-contact developing method, and its basic principle is described in British Pat. No. 1,458,766, and U.S. Pat. Nos. 3,866,574 and 3,893,418. According to the inventions shown in these Patents, a cylindrical roll whose surface carriers a uniform thin layer of toner is provided close to an electrostatic latent image surface on photosensitive material (the gap therebetween is between about 5 and 500 .mu.m) and a biased A.C. voltage is applied to the gap thereby causing the toner to vibratingly fly so as to selectively attach the toner to the electrostatic latent image portion having a potential higher than a predetermined level. U.S. Pat. No. 3,893,418 discloses a developing method wherein graduation reproducibility is selected through frequency switching on the basis of the fact that the property of a developed image varies depending on the frequency of an applied A.C. voltage.
As a result of investigations into such non-contact developing methods, it has been found that, in addition to the conventional analysis that the toner flying characteristic depends largely on such external factors as the magnitude and frequency of the applied A.C. voltage, the properties and conditions of toner itself are greatly affected by these external factors and thus it is substantially meaningless to determine the developing conditions only with reference to these external factors.
That is, it has been found that in non-contact developing systems the requirements of the A.C. voltage to be applied vary depending on the amount of electricity charged in the toner and on the particle diameter (weight) of the toner, and that the optimum frequency and voltage for the highest toner flying sensitivity also vary from toner to toner. Consequently, to compensate for variations in toner it is necessary to have some means capable of adjusting the A.C. voltage with reference to variations in the charged amount and the diameter of the toner particles in actual application. In order words, the conventional non-contact developing system which does not make such adjustments requires toner having only small variations in charged amount and particle diameter. Such toner is difficult and expensive to produce. According to the current toner production techniques, it is actually inevitable that such properties of toner vary to some extent. For this reason, toner's flying efficiency and developed result are not currently satisfactory.
Further, the conventional developing method of applying a voltage of constant frequency is defective in that, though the method can provide a high resolving power because only a narrow range of specific toner particles can fly, it has a poor image denseness and gradation reproducibility compared with the conventional magnetic brush developing method.
In order to solve the above problems, the present inventor has proposed a developing apparatus in U.S. Patent Application Ser. No. 739,878; filed May 31, 1985. In this apparatus, an A.C. voltage having a plurality of frequencies is applied during developing to compensate for variations in the distribution of toner characteristics over a comparatively wide range.